At the turn of the 20th century the airwaves of the United States were completely silent. Today they are filled to the brim with wireless communication signals. The recent history of wireless telegraphy is characterized by a concerted push towards fitting more and more information into the air. Complex modulation schemes have been developed that package information with increasing efficiency. To match this ever increasing degree of efficiency radios are required to resolve these signals with a commensurate degree of precision. Precision is required because wireless signals are corrupted by noise as they are sent through a wireless medium and because as more complex patterns are used to represent information it becomes more challenging to determine one pattern from another.
Modulation schemes such as Gaussian frequency shift keying (GFSK), Phase shift keying (PSK), and Quadrature amplitude modulation (QAM) can utilize two channels that transmit signals that are 90° out of phase with each other. The two channels are called the i-channel and the q-channel. These channels carry the in-phase and quadrature-phase signal respectively. The signal to be transmitted is mixed with an in-phase local oscillator and a quadrature-phase local oscillator to form the modulated signals at a frequency that is amenable to transmission through the air. Due to the principles of linearity, the two signals can be sent through the air simultaneously and then the received signal can be resolved at the receiver into the original component parts. This modulation scheme results in a highly efficient usage of available bandwidth.
Although I/Q channel modulation has certain benefits regarding the amount of bandwidth consumed, the increased complexity of the modulation scheme can result in several errors that make resolving the signal difficult. Like other wireless systems, I/Q channel modulated signals can suffer from carrier leak. In order to effectively transmit and resolve a signal that has been mixed with a local oscillator signal the transmitter and receiver must agree on what frequency the signal was mixed with. If the two disagree on this frequency the receiver will mistakenly interpret a portion of the mixed frequency as the information signal. This phenomenon is called carrier leak. In addition, I/Q channel modulated signals are susceptible to a group of errors that can collectively be referred to as IQ imbalance errors. I/Q channel modulation requires the receiver and transmitter to agree not only on what the carrier frequency is, but also on what the phase difference of the two signals are. If the transmitter does not impart the right amount of phase shift, or the receiver resolves the signals as if they had a different phase shift, this mismatch will also bleed into the signal itself and will show up as an error in the signal. The combined results of these errors can be broken up and expressed as four quantities; the phase error of the transmitter (ΔθT), the phase error of the receiver (ΔθR), the amplitude error of the transmitter normalized to nominal gain (εT), and the amplitude error of the receiver normalized to nominal gain (εR). The quantities can be collectively referred to as the IQ imbalance error metrics of the transceiver.
Since manufacturing processes are not perfect, the errors described in the previous paragraph will be different from part to part. This means that two parts that come off of the assembly line one after the other may have manufacturing imperfections that result in highly variant error metrics. Therefore, a designer cannot design out these error sources and produce a design that will always work. Instead, wireless radios need to be designed such that they can be individually calibrated to remove these errors.